Several newly discovered genes in prokaryotes appear to encode a group of proteins involved in iron-sulfur cluster assembly. One of these, iscU, encodes a small (13-kDa) protein that exhibits homology to the N-terminal region of NifU, a protein proposed to function in iron delivery during the assembly of nitrogenase in nitrogen fixing organisms. The exact function of IscU is not known, but it may represent an isolated functional domain of NifU, possibly serving as an iron "escort" protein. Genes encoding homologous IscU-like protein are also being discovered in eukaryotic organisms and may have similar functions in biogenesis of iron-sulfur proteins.